let a = { 0: "a", 1: "b", 2: "c", length: 3 };
console.log(Array.prototype.join.call(a)); // a,b,c
console.log(Array.prototype.join.call(a, "+")); // a+b+c
console.log(Array.prototype.map.call(a, (v) => v.toUpperCase())); // ['A','B','C']
console.log(Array.prototype.slice.call(a, 1)); // ['b','c']
console.log(Array.from(a)); // ['a','b','c']
